(i) Field of the Invention
The present invention relates to a method of heating the charge of a glass furnace.
(ii) Description of Related Art
Industrial furnaces, traditionally equipped with air-fuel burners, which burn a mixture of air and fuel, have recently seen the appearance of oxy-fuel burners, which burn a mixture of fuel and oxygen, substituting all or some of the conventional air-fuel burners. Generally, an oxy-fuel burner, in the manner of these air-fuel burners, takes the form of an individual device with localized ejection of fluids, which may frequently cause problems of temperature profiles and of the transfer of heat into the zone in question by the flame of the oxy-fuel burner.
In the glass industry, furnaces which traditionally operated with air-based burners are being increasingly equipped, partially or totally, with oxy-fuel burners. Although new furnaces may be readily designed to operate completely with oxygen (or in a general manner with mixtures of fuels and of oxidizers containing more than 50 volume % of oxygen and preferably more than 88 volume % of oxygen), current furnaces cannot be completely converted, that is to say by completely substituting the air-based burners simply with oxygen-fed burners since the combustion efficiencies of these burners are different, as are the flame geometries.
Thus, an air-based burner which heats a bath of glass uniformly cannot be replaced as it is with an oxygen-based burner which will have a tendency not to provide the same coverage of the bath of glass (for the same power).
Moreover, glass furnaces generally include regenerators (or recuperators) which make it possible to recover the heat from the hot gases of the flames before they are evacuated from the furnace. In the case of "end-fired" furnaces, there are usually two of these regenerators, placed in the upstream part of the furnace close to the zone where glass is fed into the furnace. In the case of cross-fired furnaces, the regenerators are located on each side of the furnace. In both cases, they are in communication with the furnace via openings called "ports", one port for each regenerator in the case of end-fired furnaces, regularly distributed along the side walls of the furnace, these ports being uniformly distributed along the side walls of the furnace in the case of transverse [sic] furnaces. Usually, fuel is injected below these ports from which are expelled air jets preheated by the heat of the regenerator.
In the case of cross-fired furnaces, the smoke from the flames formed are evacuated via the ports located in the opposite side wall, which also includes fuel injectors. The flames are regularly inverted (for example every twenty minutes) so as to heat the regenerators (which preheat the air sent into the furnace in order to create the flames, thereby saving energy) alternately.
In order to substitute the air (coming from the "port") and fuel (coming from the injectors) assembly with an oxy-fuel assembly, the most obvious solution consists in placing an oxy-fuel burner at the place of one of the fuel injectors, and in no longer using the other fuel injectors and in no longer using the influx of air via the port located above said injectors. The length of the oxygen flame thus obtained will be roughly equal to that of the air flame created previously, while at the same time replacing the power of the air-based burner.
This solution has the disadvantage of considerably reducing the coverage of the bath of glass by the flame. This is because the air flame, for example, with a width of approximately one meter to one meter fifty, is replaced by an oxygen flame with a width of approximately 40 centimeters.
In order to solve this reduction in coverage, it may be envisaged fitting not one but two burners of half power arranged side by side.
This solution has the advantage of allowing better coverage, widthwise, of the bath of glass by the flames but it has the major drawback of shortening the length of flame by approximately 40%.
In order to obtain an oxygen-based flame as wide and long as an equivalent air flame, the invention proposes using two oxy-fuel burners placed side by side and carrying out so-called staged combustion with respect to these two burners by adding a lance between the two burners and by distributing the oxygen between burners and lances. By preferably allowing only approximately 60% of the oxygen necessary for complete combustion of the flow of fuel into the burners and by transferring the approximately 40% remaining to the lance, it is possible to produce an oxygen flame having the same length and width characteristics as the air flame.
Staging of the combustion also makes it possible to reduce the temperature of the flame and to decrease the production of nitrogen oxide NOx.
From application WO 90/12760, it is known to increase the efficiency of a glass furnace operating with burners fed with air and a fuel by adding to the furnace several oxy-fuel burners, usually two oxy-fuel burners in the furnace-charging zone as well as two oxy-fuel burners in the glass-refining zone, so as to increase the thermal energy transferred to the glass.
However, such a furnace "boosting" system is added to the existing burners, which increases overall the energy supply.
In practice, such a solution cannot easily be applied since glass furnaces do not possess in general additional port [sic] via which new burners might be run into the furnace.
U.S. Pat. No. 5,116,399 describes a similar solution, in particular for furnaces of the end-fired type ("v or u" type flame, end-fired regenerative furnace) in which an oxy-fuel burner having a high flame momentum (velocity greater than 100 m/s) is placed in the front wall of the furnace, in the refining zone, so as to supply more energy to melt the glass and to keep the unmelted ingredients in the melting zone, by virtue of the high momentum of the flame. The additional drawback of such a method is that the high-momentum flame causes particles of unmelted glass to fly off throughout the furnace, which is prejudicial in the case of polluting dust emissions.
The commercial brochure entitled "GLASSMAN EUROPE '93--Industrial Experience with oxy fuel fired glass melters --presented at Glassman Europe 93--Lyon--France--Apr. 28, 1993" describes the boosting of glass furnaces, either of the cross-fired type or of the end-fired type, with regenerators or recuperators, using one or more oxy-fuel burners placed in the furnace, in addition to the existing air/fuel burners in the furnace, so as to increase the efficiency of the furnace.
All the abovementioned documents relate to the boosting of glass furnaces with additional oxy-fuel burners which are assumed to increase the energy transferred to the glass. However, none of them relates to the substitution of air-fuel burners with oxy-fuel burners and problems to be solved in order to obtain a glass furnace of superior efficiency with a reduction in fuel consumption.